Move dart2js from sdk/lib/_internal/compiler to pkg/compiler

pkg/compiler/lib/src/constants/expressions.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.constants.expressions;
 
 import '../dart2jslib.dart' show assertDebugMode;
 import '../dart_types.dart';
 import '../elements/elements.dart' show
     Element,
@@ skipping 392 matching lines @@
   visitConditional(ConditionalConstantExpression exp) {
     write(exp, exp.condition, leftAssociative: false);
     sb.write(' ? ');
     write(exp, exp.trueExp);
     sb.write(' : ');
     write(exp, exp.falseExp);
   }
 
   String toString() => sb.toString();
 }